text:
Why did air traffic levels decrease dramatically in the US through the 2000s?
In 2015, the FAA stated that

System-wide, air traffic has declined by 24 percent since peak year 2000. The chart in Figure 1.1 shows that air traffic
  volume is not expected to return to peak levels in the near term.

This is talking about traffic in terms of ATC operations.
My question: why was there such a drastic, sustained drop-off? I understand that commercial traffic dropped off after 9-11, but I assumed it had since returned to 'normal'. Apparently this is not the case, but are there any other reasons? Is it something with the metric? It doesn't seem like there are any less people flying, do airline fleets consists of larger jets than 15 years ago?
(Info from the Air Traffic Controller Workforce Plan )

Just taking a guess (thus a comment): Since this graph shows ATC operations and not passengers, would it have anything to do with consolidation within the industry? It could be that there is now one plane (carrier A) flying between AAA and BBB where there used to be 2 or 3 planes (on carriers A, B & C). It would be interesting to overlay this with actual passenger numbers to see if those have remained somewhat more constant or have recovered from the 9/11 and 2007-09 depression dips better.

@FreeMan number of passengers is decently correlated: http://www.rita.dot.gov/bts/press_releases/bts016_13
but note that the OP graph includes non-commercial traffic.

@Federico That link doesn't show prior to 2006, though, so it misses the peak shown in the OP's graph.

@reirab that's why I said "decently" and not "extremely well"

Now correlate that chart to the S&P 500 and the Baltic Dry Index...

This question (and all of the answers) assumes air traffic activity = commercial passenger flights. It doesn't! There's cargo, general and corporate aviation to consider. Recreational general aviation in particular was hit by high oil prices in 2008.

Your air traffic controller's metric doesn't mean anything to most people. Here is what actually happened as measured by passengers carried, and it looks nothing like your graph.

The 911 attacks in the US caused a blip after 2000, and the world financial crisis caused another after 2008, but passenger growth has been growing exponentially since deregulation started in the 1970s. The answers trying to explain the graph as reduction in demand are not correct.
Aircraft size and frequency now match demand, and they are packed full. US domestic load factors went from 70% in 2002 to 85% in 2017, an 18% increase. Ryanair is hitting 95% in Europe. Aircraft are also getting bigger, or at least seats are getting packed closer together. MIT saw US seat capacity per departure increase from 165 to 178, another 8%. Ryanair only flies aircraft with 189 seats, and new aircraft will cram in 197.
Cheaper fares, lots of travelers, and a lot fewer empty planes flying government mandated routes. Since your graph only indicates a 20% drop in aircraft movements, this explains much or all of the difference.

While a gravedig, this is the real answer. Even the 2001 and 2007 dibs just caused global demand to flatline (NB: US demand plummeted, ROW kept growing. The US is much less important to the world than the TV would have one think.) More people and much more cargo are flying today than ever. Long-distance driving is largely a US thing; even if it's marginally cheaper to drive a full car than to fly, the added time cost well exceeds the lost income of anyone who has the need or the money to travel a long distance.

The question was about US traffic, while this graph gives worldwide traffic. This one for only US traffic gives a different picture (there were actually 2 dips), even though in the end the trend is still upward for the reasons you gave.

I think just replacing the worldwide graph with the US version would make the comparison to the OP’s graph a lot more valid. Your call.

Isn't the difference in the declining GA traffic? Both answers focus on revenue flights but the graph includes GA Traffic, and that has most definitely declined.

@PeterKämpf You might try putting this in an answer, but from what I know GA operations in the US only totaled about 8m in 2000 and were still above 6m in 2018 while the OP's graph shows a 35m decline in ops.

@Pilothead I was just guessing, thank you for the figures! So I am left wondering what the discrepancy between the plots might be.

It isn't just one thing, but a vicious cycle:

Airline travel is expensive. One person flying and one person driving end up about the same price once you figure in gas and hotels, but the airline will want that same price for each passenger, while you can fit up to three more people in the same car at practically no additional charge (the extra weight will slightly increase gas costs and at some point everyone you can put in a minivan won't fit in one hotel room, but the incremental costs still don't compare at all). This makes time practically the only advantage to flight, especially for a family group; if you have the time to drive, many people who need to get somewhere don't even consider flying as a viable financial option.

You choose your car, among other things, for a certain level of comfort; the airline sets up their planes for volume over all else. Even the smallest sub-compacts have more legroom (for the front seat passengers at least) than the average economy airline seat. Upgrade to first class and the ride's better, but then the cost doesn't compare at all.

Customer service ratings are at an all-time low. Flight attendants are moving further and further from the "stewardess" role they had in a bygone age when airlines lived and died by their service quality (because regulation gave them little control over anything else in their business model). Now they exist for little else than to make sure nobody dies, or worse, breaks Federal aviation laws. Get them on the plane, give the speech, make sure everyone's belted in for take-off and landing, then get them back off the plane, all without killing themselves or each other. Anything else the the flight crew does during the flight is gravy at this point.

The major U.S. carriers have practically all adopted Spirit's business model. The average person considering air travel today might think Spirit's the only airline that charges you extra for every single additional amenity or service, but in fact Southwest, a "discount" carrier, is the only one that still rolls 2 checked bags, snacks and drinks into the ticket; American, United, Delta, USAir, they all charge \$25 for the first checked bag and \$50 for the second, and most of them are also charging for any food or drink served on board.
This is due to the hypercompetition on price, which is partially our fault as consumers. In a related but different industry, StubHub recently abandoned its "all-inclusive" pricing model, because it saw its sales drop by half due to StubHub being the highest price in most side-by-side comparisons. It's the same with the airlines; if you need a flight, you go to Google or TripAdvisor or Expedia, enter your route and the date window of your trip, sort by price, and pick one of the top three, maybe the top five. That means that anything the airlines can do to lower the price they show you online at first glance gets them the customers, and airlines wanting to keep the all-inclusive model, like Southwest, have to aggressively remind customers that what they see online is what they will end up paying when it's all said and done (hence the "Bags Fly Free" ad campaign that is now painted on their planes' cargo hatches).

Security is a nightmare. I remember a time, not that long ago really, when you put your carryon or purse on the conveyor belt, your coins in a dish, and walked through a metal detector. Simple. You didn't even need a ticket to get into the secured area, so you could meet whomever was flying in at their gate instead of at the curb. Those days are gone, and it will take a couple big technological leaps to get anywhere near that again (like being able to walk through a full-body scanner with your shoes and all your bags, instead of adopting the "felony stop" position in stocking feet with the contents of your briefcase laid out in grey tubs).

The expense of any kind of travel is redefining the idea of a "vacation". Leisure time overall has been decreasing since the advent of the Information Age with everyone interconnected and instant gratification being practical in more and more aspects of our lives. Those who still use their two weeks a year (if they even get that) for real leisure are opting more and more often for a "staycation", either staying home relaxing and visiting local attractions, or driving a relatively short distance to a hotel or resort (DFW has two big destinations, the Gaylord Texan and the Great Wolf Lodge, which cater primarily to people who already live in the Metroplex).

Video calling has created a difference between "face to face" and "in person". They used to be synonymous, but now the kids can not only talk to Grandpa, but see him, and he them, over a mobile phone or a computer with a webcam, at virtually no cost beyond the telecom plan. This development has more thoroughly revolutionized business travel; you no longer need to fly to the corporate office to get face time with an executive or have a big corporate meeting; you just sit down somewhere with a computer, camera and microphone.

All of these things have deterred all but the most necessary travel by air; getting on a plane is so expensive, so dehumanizing and so uncomfortable that it's only done if you have to get somewhere and can't take the time to drive there (or you have to cross a major body of water to reach it). That has reduced total passenger count, and since a flight only makes a profit if it's very near full, that reduces the number of flights airlines make. However, that decreases total revenue to offset the overhead of the airline (marketing, scheduling, maintenance, etc), so airlines cut costs by slashing services, making a flight on their planes even more miserable, reducing the general desire to fly, reducing passenger count. The vicious cycle continues.
This reduction has been offset somewhat by U.S. airlines "rightsizing" their fleets, opting for more flights with smaller planes. This gives passengers their choice of flight times while keeping planes full and pilots/flight attendants employed. Boeing is focusing primarily on this market in new plane orders with the smaller, more efficient 737, 777 and 787, while the original "jumbo jet", the 747, is on its way out of most passenger fleets and the A380 is struggling outside a couple relative niche buyers like Emirates and Qantas that specialize in ultra-long-haul flights.
Increased air cargo flights have also made up some of the difference; boxes don't care nearly as much about being stacked like sardines in an old aircraft (when was the last DC-10/MD-11 you flew in as a passenger? UPS still has dozens of them in the cargo fleet) with minimal amenities, so thanks to the success of e-commerce, FedEx and UPS are raking it in and growing their air fleets (though they passed on the A380 in part due to Airbus not prioritizing the cargo variant), while American is scheduled to reduce its fleet by 30 planes this year and is still struggling to report a profit even with prices and services cut to the bone.

What do you mean by "Airbus can't keep up with A380 orders?" If you mean that Airbus is getting a lot of A380 orders, that's very incorrect. The A380 is having a very hard time selling right now, just like the 747. Their CFO recently stated (this year) that they may shut down production on the A380. Boeing was right. Airbus basically already acknowledged this by releasing the A350 (which is selling very well) and the A330neo (which is also selling well.) As of the end of May, exactly zero A380s have been ordered in 2015.

The most recent information I had was that Airbus had 317 orders for the A380 and had delivered half of those as of the end of last year. It had also taken orders for the A380F that it was ultimately unable to fill because of prioritization of passenger-variant production. The biggest problem the A380 has had is that Airbus simply cannot build them fast enough to deliver orders in a timely manner; at current production rates of 45 aircraft a year it has about a 3-year backlog, and Airbus only got to that rate 5 years into production.

Yes, there have been 317 total orders, but most of those are old. Additionally, several customers, including Air France-KLM, have indicated that they will cancel some or all of their remaining orders in favor of smaller aircraft (i.e. probably the A330neo or A350 in the case of Air France.) See this article. According to Airbus' own CFO, if they're unable to get enough customers to commit to buying a re-engine A380, they'll likely stop production within a few years.

The Dreamliner's had its startup problems as well, but it has an advantage over the A380 in the U.S. market for another really good reason the A380 is failing; the 787 matches the U.S. domestic carriers' model of smaller jets running more flights. Passenger VLAs only make business sense for ultra-long-haul flights where you only want to run one or two a day (which is why Emirates and Qantas love their A380s; their flight schedules include some of the longest nonstops in the world). Even then, the 777 ER is the plane of choice for 22 of the 30 longest flights in the world.

Apparently, I was off by a month, though. It was Dec of last year that Airbus' CFO indicated that they may have to cancel it... and they haven't had any new orders since that time, making that outcome seem increasingly likely.

As for 787, yeah, it had some startup problems, but it's selling exceptionally well. In the span of a decade, it has accumulated 1,105 orders. 777 is also selling extremely well, as are A330 and A350. 747 and A380, not so much. Delta recently chose to order a mix of 50 A330neos and A350s just because it couldn't get more 787 production slots soon enough.

I think we're speaking the same language. A380 demand was initially higher than Airbus could meet, but to your point it's fallen off. Airbus had predicted a move by airlines to fewer flights with larger planes to reduce flight crew staffing and relieve congestion at major airports. That market has not materialized; airlines have favored smaller, more efficient aircraft and more flights. An airline can cancel one flight in ten if the route is underbooked; if it only schedules two or three flights a day that flexibility really doesn't exist.

Yeah, the massive order by Emirates probably made the A380 seem to be selling somewhat better than it was, though. 140 of the 317 orders were just Emirates. I still can't imagine what they plan to do with all of the wide-bodies they have and have ordered. In addition to those and the tons of 777s they already have, they've ordered 150 777Xs.

Well as I mentioned, the Emirates flight schedule is practically all trans-con; there are a few hops to Riyadh, Kuwait, Tel Aviv etc. but beyond that you're looking at flights from Milan, CDG, Heathrow, Moscow, Beijing, etc, and then they get really long (IAH, SFO and DFW are on the top 30). As far as why, this all feeds into the UAE's plan; to transition from oil wealth as a primary economic driver into high-end tourism, business and retirement; they want to be the Orlando of the Eastern Hemisphere and it seems to be working.

Most of this answer is pure speculation and the arguments given for a general decline in passenger air travels seem to be contradicted by Pilothead's answer.

People travel by air for mainly two reasons, and both require money:

business
pleasure

Since the source of that chart is FAA, my answer is mostly related to USA, but it can generalized globally.
Economy
Years leading to 2000, there was an economic boom. Air travel was increasing. By 2000-2001 (before 9/11), dot-com collapse resulted in a recession. This forced companies to lay offs and not to pay for air travel.
Economy started to get better after that, and then the recession of 2007-2008 hit. Companies learnt an even bigger lesson not to spend money on luxuries.
Economy started to get better again. Air travel started to increase again, but did not peak and is not predicted to peak to the same level as of 1999-2000.
Why? After a financial calamity, companies and people are very reluctant to adopt same spending behaviors as before the calamity, since there is an uncertainty of it happening again.
Airline Mergers
After 2000, airlines was one of the industries which was hit the hardest. So they wanted to maximize their profits. That means that all airplane seats must be occupied. This resulted in fewer flights, and also increasing codeshares.
Many airlines acquired other airlines. In the US alone, several major airlines merged together, eliminating redundant and unneeded flights. This reduced air traffic too.
General Aviation
Also with economic issues, people started to spend less on their hobbies. As a lot of GA traffic occurs on airports with ATC, this reduced ATC operations too.
Larger Airplanes
As Federico has mentioned, only A380 is the larger aircraft so this reason is not likely to contribute for decline in air traffic. On top of that, A380 is mostly used on international routes whereas the majority of air traffic is domestic.

You might also consider the growth of alternatives to business travel, such as cheap and easy access to video conferencing over the internet.

In "This resulted in fewer flights and codeshares", I assume "fewer" is supposed to apply only to "flights". If so, it would be much clearer to write "... fewer flights and more codeshares." Also, I'm not sure that it's relevant that the A380 is the only new big plane in the period. Replacing a regional jet with a 737 would still be a case of using larger planes, even though a 737 isn't particularly large.

Lots of assertions here...some quantitative info would be nice ;)

@digitgopher Assertions? Quantitative info? I talked about two slumps we had in the last 15 years, and several airline mergers. I linked the Wikipedia articles for details about those events. I had witnessed both recessions first hand. What do you need as quantitative info?

@Farhan Some examples: "fewer flights"...numbers on this? "several major airlines merged together..."...exactly how many and are there any documented effects? Overall I like your thoughts, they are helpful.

@digitgopher As far as the major mergers, there were 3 major ones and a few other relatively large ones. The major ones were Continental + United, Delta + Northwest, and AA + U.S. Air. Others included U.S. Air + America West and Southwest + AirTran.

@digitgopher I do recall seeing numbers to back up the claim that occupancy percentages are indeed much higher now than they used to be (and this has been my personal anecdotal observation as well,) but I don't have a good source for those numbers off hand.

Another factor with GA is the increased security after 9/11.  Before that, we kept the plane on the GA side of a commercial airport, and could drive out to the ramp to load (and leave the car there) or work on the plane.  With security you couldn't do that, and even getting to the plane was a hassle, so we moved to an uncontrolled field, even though the drive was 20 minutes longer.

I think all the firefighters, charter pilots, 'freight dogs', flight instructors, paramedics, photographers and other GA professionals would prefer not to be described as "hobby" pilots :-)

@Pondlife but I was only talking about those who do it as a hobby. :)

As you can see the steepest drop is not around 2001, but between 2007 and 2009, the years when the current financial crisis began, it is reasonable to assume that many passengers remained home because of that.
Concerning the size of the aircraft, except for the launch of A380 in october 2007 with Singapore Airlines, I would say that it hasn't increased significantly over the last 15 years.

2008-2009 does match nicely with the finance graphs, but the rest doesn't really correlate too well.

@digitgopher It is not given that once finance starts moving up, everything else follow suit. You should compare to average expenses per household (or along such lines) but at that point I think you would be better off on Economics.SE

The financial crisis doesn't explain the 13 year trend. And why would you say the size hasn't increased significantly? I would be happy to upvote a more detailed answer.

@digitgopher the financial crisis explains the biggest drop, as you also agreed upon, I made no mention of the trend. The size hasn't changed because new ariplanes are not bigger than the old ones, except for the A380.
Not upvoting I can understand, downvoting means that there is at least some mistake.

@Federico Actually, lots of newer airplanes are substantially larger than old ones. Even some of the new regional jets are almost the size of old 737s. New 737s are almost the size of 757s (737-900ER.) 777 also grew substantially with the -300ER and -200LR. 747 grew substantially with the -400 then again with the -8.

The fact that you can't buy bigger widebody aircraft doesn't mean the average plane isn't being upgraded in model. Over the past 30 years, the typical regional plane has risen from the 19-seaters (Beech 1900), to 30-seat props (EMB120s), to 50-seat jets (CRJ200), and now those are being pushed out to 70-100 seats (CRJ900, E-Jet). That's a 5-fold increase.


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How does an airport catch up when it is behind on traffic?
When airports fall behind traffic for some reason and delays piling up, how do they catch up? If flights need to be cancelled, who decides (and when) - ATC, airport management, or airlines themselves?
As an illustration, here is a snapshot from this page (taken at approx. 2355 UTC 12/23/15):

there's usually a bit of margin built in the schedule to account for emergencies and such.

Most airports aren't at 100% capacity any of the time: they simply fit in a few extra flights for a couple of hours until they're back on track, causing relatively little delay.
Even those who are at 100% capacity sometimes, aren't usually at 100% capacity 24/7. They can recover by delaying everyone a little for a few hours, with the delay slowly reducing as the airport gets quieter

With a very busy airport (Heathrow is often quoted as an example), however, it's possible that the airport is at/near 100% capacity for long periods. Heathrow runs at nearly 100% capacity (roughly 98%), and much of the "quiet" period is actually at the very start of the day which doesn't leave much slack.
However, there's some scope for "absorbing" flight delays: at Heathrow, most aircraft will sit in a "hold" stack for a while before landing. Aircraft join this stack as they arrive and wait their turn to land. If one plane is a little late, another will simply take its place in the queue and everyone jumps "up" one space until the delayed flight. As long as there are aircraft in the stack, short delays or aircraft arriving early don't really make things too chaotic - there are still 60 aircraft landing in the next hour, it doesn't really matter what order they land in.
However if there's a bigger problem such as ice, which means the overall capacity is reduced, flights must be cancelled as they simply cannot all fit in. In this case the airport will typically reduce capacity across the board and allow the airlines to choose which flights to cancel. In the case of Heathrow, this will usually be the relatively local, high-frequency routes: eg the once-a-day flight to Sydney, Australia will still fly, but one of the once-per-hour Glasgow flights will be cancelled. For the most part the number of cancelled flights is down to the airport/ATC, but the specific flights cancelled are up to the airlines.

Well, the delays usually pile up during the busy hours. They manage to eliminate the delay times when the flights arrive or land at the non-busy hours which dissipates most of the delays. One or two flight cancellations won't have much impact on the departure or arrival time during busy hours because the airport is packed.
This situation is very similar to rush hour traffic on roadways dissapating after those hours.
To answer your second question, it is mostly airlines that cancel their flights. Airport management might cancel a flight if there is something really wrong with it. and ATC might cancel if the weather or some other occasion warrants it.

To add to the other answers, one thing ATC will do is order ground stops. Basically they stop the inbound flights at their origin until there is space to fit them in at the destination.
There's an excellent blog post on the whys of ground stops and why planes will leave the gate and sit on the ramp sometimes for hours.
When there's congestion or delays at the destination, ATC will often give them a narrow opening to get in the queue. If they can't be in the air in so many minutes then someone else gets the spot.
It's a very informative read. He's also got a post on how the "passenger bill of rights" throws a wrench into this.

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How is passenger baggage transferred, and how does it happen so quickly?
I've always wondered how airlines transfer bags between connecting flights. At a hub, bags must be moved to any number of connecting flights throughout the airport - some with very short connecting times. Seems like sorting them would take a lot of time. How does this process happen?

Possible duplicate of: What is the average time taken to load and unload the luggage?

@kepler22b It depends on the person offering the bounty. This way, the question gets more attention than other active questions.

This belongs on Travel.SE.

How? Lots of practice, and very little care taken not to damage the luggage.

On big airliners, baggage is mostly stowed in containers, called Unit Load Devices. They are packed at the departure airport and pre-sorted to minimize the amount of sorting at a hub, but normally need to be emptied after arrival so the baggage can be re-sorted.
On regional jets and older aircraft which don't take ULDs, bags are still loaded individually and transported to and from the aircraft on open carts which are assembled to small trains (see picture below).

Airport baggage transportation (picture source)
Once the ULDs or the carts arrive inside the terminal, the bags are unloaded into the baggage handling system, a collection of conveyor belts, barcode readers and switch points. Since every bag received a sticker with a big barcode upon check-in, the reader can report the barcode of the next bag to a computer wich sets the switch point accordingly. Traditional systems employ humans with a portable barcode reader for putting the bags on the right belts.

Baggage tags (picture source)
The most recent systems put the bags on plastic trays which have RFID tags attached, and the RFID information is used for switching the conveyor belts. By loading the bags on standard size trays, the flow of bags is simplified. However, a baggage handling system is a complex piece of machinery, and some new airport terminals were delayed because the system did not work initially, or opened with lots of mis-routed or even mangled bags in their first months of operation. London's Terminal 5 and Denver airport, which went back to manual sorting after 10 painful years of trying to get their system working, come to mind.
When they are sorted, the bags are deposited either at a ULD packing station or an empty cart for transportation to their respective flight. At the end of their journey, they are routed onto baggage carousels for pick-up by their owners.

Baggage carousel (picture source)

the source of your first picture doesn't show up

I would add that bags are still bulk loaded on many narrowbody aircraft, not just regional jets.

@ZachLipton: Agreed, I edited the text accordingly.

@ratchetfreak: Thanks for checking. I forgot to place the link. Fixed.

Why don't regional jets, even with newer planes don't use ULDs?

@Firee: Their baggage holds are too small - narrow fuselages have disproportionally less cargo space.

@PeterKämpf: Is there a list of aircrafts which are compatible with ULDs.

In case of connecting flights in same airline or between partner airliners which allow baggage transfer, the baggage is usually checked in for the duration of the journey i.e. from source to destination.

In case the time between interconnecting flight is short, someone (usually airline agent) picks up the luggage from the the aircraft and loads it into the connecting flight. If I understand correctly, such cases are marked in their tags so as to allow for proper storage in cargo hold (for easy retrieval).

Transfer of luggage into a flight, from denverpost.com.
As this forum says,

Most US airline tags will either be a 'city', 'hot' or 'cold'. City is a bag going straight to where the flight is going. Hot and cold are bags that are connecting onto another flight, depending on the amount of time between flights. Hot bags are usually loaded separately and are the last ones loaded onto the flight so that they are the first ones off at the destination and they can make the connecting flights.

Source: australianfrequentflyer.com.au
For example, SWA put a single driver in charge of collecting all the baggage for a particular outbound flights from all the inbound ones.

In case there is sufficient time between connecting flights, the agent (who has a list of baggage to be transferred) unloads the baggage and loads it into the airport baggage control system, which then sorts it automatically into the correct aircraft.
Having the bags in ULD (unit load device) operates in the same way. Each bag bar code tag triggers the opening/closing of a "door" on its way so, after a few branchings it arrives into a "sorter" which sends it into a particular container.

Of course, this assumes that there is no security checks involved in the middle (like in international flights). In that case, the system is similar, but involves screening.

Preamble
I will keep it short and try to hit the point since I see some good answers here, please feel free to comment to get more detailed information
Answer

How does this process happen?

In airport baggage is handled from the check-in desk to the Unit Load Device  (ULD) via a Baggage Handling System. In about 15 minutes the baggage is automatically sorted, thanks to some sensors that scan the bag tag attached to your luggage. In the meanwhile the baggage undergo X-Ray and potentially other security checks. It finally gets to the dedicated "carousel" (similar to the one where you retrieve your baggage). Here the operators put it in different ULD. Transit baggage are usually kept in a different ULD so that they are the first to be unloaded from the aircraft and can be quickly sorted, and re-directed to the correct connection flight.

How is passenger baggage transferred?

There are three different technologies, and here you can have a detailed look at them:

Tilt trays where each baggage has its own tray and the tray is moved around.
Convey belts, similar to the one you see in most airport
Operators: the baggage are moved by hand by an operator and a dolly

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text:
What is fly-by-wire?
What exactly does the term "fly-by-wire" mean? Where did it originate? I understand that it refers to control systems. If an aircraft is not fly-by-wire, what is it? That is, what are the alternatives?

The fly-by-wire means that in the aircraft, the (pilot or autopilot) control inputs are fed to the (flight) computer, which processes them and determines the required control surface movements and transmits this by electrical signals (through wires, hence the term fly-by-wire) to the appropriate actuators.
This is in contrast with the mechanical system, where the pilot control inputs are directly transmitted to the flight control actuators, through push-pull rods or cables. The following figure shows the schematic of a mechanical and fly-by-wire system.

Source: euroaviasevilla.es
The first aircraft to use this mode of control was the Avro Arrow, which used a dual-channel fly-by-wire system made of analog circuits. The flight control mechanism on the Apollo Lunar Module was fly-by-wire, the Primary Guidance, Navigation, and Control System, which was referred to as PGNCS, pronounced 'pings'.
Later, NASA tested the fly-by-wire system in a F-8 aircraft, a schematic of which is shown below. Note that it uses Apollo flight computer.

Image from Computers Take Flight- A history of NASA's Pioneering Fly-by-wire Project
In civil aviation Concorde and A320 (which had a digital-fly-by-wire system) introduced fly-by-wire systems, while in military, it was the F-16. At present, almost all combat aircraft and large airliners use fly-by-wire systems.
As an extension, the system is called fly-by-light (or fly-by-optics) is the signals are transmitted through optical cables. If the wires are eliminated altogether, it is called fly-by-wireless.

Not all FBW systems have a computer to interpret the input and provide adjusted output. Some just use wires to transmit the pilots intent to the control surface actuator instead of cables/rods.

@FreeMan Sorry, but that's not actually true. "Fly-by-wire" is used to specifically to refer to control systems where the flight control surfaces are actuated by a computer that interprets the pilot's inputs. If the controls are electrically connected without computer control, it would probably be called "servo actuation", "fly-by-electricity", etc. (That control scheme not being common enough to have its own snappy name.)

Fly-by-wire means that the control commands travel by wire to the control surfaces. There, either electric motors or hydraulic actuators do the physical work of moving the surfaces. The pilot has either a conventional control column or a side stick with transducers which will encode the commands of the pilot into electric signals. Optionally, a computer can be employed to modify the signals such that overload conditions or dangerous maneuvers are avoided.
Other control systems can be summed up as mechanical: Some pushrod or steel cable runs all the length to the control surface. To relief the pilot of the mechanical work, those pushrods can be boosted by hydraulics, so the mechanical movement of the control column or stick will become a command for the hydraulic actuator, similar to the electric signals in a fly-by-wire system. Early systems used a variety of aerodynamic means to lighten control forces, and small aircraft have low enough control forces that they can be flown with a direct, unboosted mechanical linkage. In some cases, it is even necessary to artificially increase control forces to give the pilot enough feedback.
Russian combat aircraft prefer to use pairs of steel cables over pushrods because they will be more resilient when hit by a bullet. Also, most early aircraft used cables for weight reasons. Today, pushrods are preferred because they allow a stiffer linkage and less friction.
Regarding the history of fly-by-wire, I could just copy the Wikipedia entry, but I prefer to link to it here.

The irony is that airplanes that are not fly-by-wire are fly by wires... Steel cables that is.
Earlier planes (and many smaller GA planes to this day) typically use steel control cables that directly connect the control surfaces to the joke or stick. Such as can be seen running to the tail section of this plane

(source)
This is not limited to small planes by any means, many larger planes use control cables as well. You can also use control rods which is common place on Mooney aircraft however those generally come at the cost of weight but may provide a more tactile feel.

Fly-By-Wire generally means that the control inputs are translated via electronic cables to a set of hydraulic actuators to move the control surface. To expand on that, FBW systems also decide what movements the control surfaces will make and often do not allow the surfaces to move such that it will put the plane in a dangerous attitude. Simple electric signal control of actuators is not necessarily FBW and even planes as simple as the Cessna 172 have electronically controlled flaps that are not considered FBW.

The fundamental idea behind fly-by-wire is for the aircraft control response to be predictable, desirable, and safe.
Modern commercial aircraft designs increasingly abandoned conventional pushrods and cables in favor of fly-by-wire (FBW) architecture, simply out of competitive necessity driven by performance and safety features. Direct mechanical connections to the flight controls are simple and reliable for small/light aircraft, but will have problems in larger aircraft with higher performance capabilities. Mechanical flight control systems have increasingly undesirable characteristics at higher gross weights and speeds, lacking both maneuverability and cruise efficiency.
Larger aircraft have much greater inertial effects, therefore needing larger flight control surfaces, which also must be located farther from the cockpit controls. This stretched arrangement over long distances causes increased friction in the control cable paths and also high control forces at increased speed. Low speed flight of a large aircraft requires equally large control deflections driven by exaggerated pilot control movements. Precise control becomes degraded as every mechanical connection in the control path from the pilot’s hand to the elevator can wiggle, even just a little bit. The freeplay, or slightly loose connections along the length of the control path, add up to increase the control “deadband” for the pilot’s controls around the neutral, centered point. Both high- and low-speed effects make precise control more difficult to engineer for mechanical flight control systems, leading modern aircraft designers to implement fly-by-wire systems.
The simplest implementation of a fly-by-wire system needs the following components:

a pilot interface to sense control inputs (for simplicity, called the “stick”)
a flight control computer to translate pilot inputs to control surface movements
servos to position the flight control surfaces
electrical power for the electronic components
a high-power system for the flight control servos

This simplification of a fly-by-wire system is functional, but for such a simplified system its utility is probably limited to a radio-controlled model aircraft. Each component operates without backup, meaning that any single failure results in a loss of all system capability. If a sensor in the pilot interface fails, the aircraft will crash. If the flight control computer loses power or freezes, the aircraft will crash. This simplification of a FBW system is straightforward, but fragile. There is no feedback to the pilot, so correct function of each component can only be assumed, and any failures will not be identified for correction. Without some redundancies and other sophistication, it will lack some capabilities needed for a safe and reliable full-sized aircraft.
Some of this is excerpted from my book, “Introduction to Fly-by-Wire Flight Control Systems: The professional pilot’s guide to understanding modern aircraft controls”.


text:
What is the difference between "R/C", "Drone", "UAV"?
Are "remote controlled" aircraft and "drones" the same thing? Are they synonyms for "unmanned aerial vehicle"?

This could be one for English.SE too

If the differences are all subtleties of the English language (and/or negligible) that could be an answer I guess. I was thinking they might have different, well-defined meanings.

Drones are simply the common name for Unmanned Aerial Vehicles. Representatives from the industry and the military use the term UAV for the aircraft and UAS (Unmanned Aerial System) for the whole system- including communication system, on ground controller etc.
In general, technical publications call them UAVs or usually UAS's and rarely (if ever) drones. On the other hand, the mainstream media refers to them usually as drones as it is easy to communicate.
The ICAO (International Civil Aviation Organization) calls them remotely piloted aircraft. However, there is a subtle difference in using this word as it means that it is being flown by a pilot (controller) on ground.
Though this may be technically true, most of the UAVs, at least those used in the military are capable of highly automated mission profiles (in fact, they depend on the controller only for weapons release) and are on the way to full autonomy.
The choice of wording is one of semantics i.e. about the meaning which is to be conveyed. For example, the nobody wants a system that goes and kills someone without human intervention. So, the USAF calls the MQ-9 Reaper a Remotely Piloted Vehicle in its official page and a UAV in press releases, but not a drone, as it has negative connotations in public mind.

Actually, the USAF more commonly uses the term "Remotely Piloted Aircraft", or RPA, more than RPV


text:
Is there an efficiency measure for airports?
For large, busy airports (such as JFK), how do they measure how well they are being utilized? I assume airport management wants to serve as many flights as possible, for maximum revenue. Any time their gates are empty, or runways aren't being utilized, the airport has more capacity to sell. Is there some sort of industry standard measurement for "airport utilization efficiency"?

The Air Transport Research Society (ATRS) has a benchmarking system for determining the efficiency of the airports.
Basically,it takes the airport size and related infrastructure (runways, no. of gates etc.) as inputs and measures the airport efficiency in terms of aircraft and passenger movement,revenue etc.
There is a yearly Global Airport Benchmarking Report.


text:
Are there differences in air traffic services between western nations?
What are the major differences (if any) between the air traffic control service in Europe vs. USA vs. Canada?

The Canadian ones say "eh" a lot.

Related: http://aviation.stackexchange.com/questions/1210/what-is-the-difference-between-icao-jaa-faa-easa-and-caa and http://aviation.stackexchange.com/questions/30158/where-can-the-faa-versus-icao-official-filed-differences-be-found

One of the big differences between regions is how aircraft operators pay for the service. In the US, the FAA is funded by taxpayers and fuel levies. In New Zealand (for example), aircraft operators are billed for use of air traffic control services. I don't know what the billing situation is in Europe.

In the UK, en-route ATS is funded by charging the airlines.

Europe, being Europe, lets each country set their route charges, but has at least managed to unify the billing process. It is managed by Eurocontrol and is explained in more detail here: https://www.eurocontrol.int/faq/route-charges

Very, very few. We (not just the West, but most of the world) all follow ICAO document 4444.

Just as a matter of historical oddities, though Saudi Arabia and India followed ICAO technically back in the 1990s you had to be religiously correct if you were talking to a religiously-sensitive controller. In Saudi, if you didn't start your initial transmission with "salaam alaikum", they would not acknowledge you. Also in India when Hindu nationalists started advocating that the name of Bombay be changed to Mumbai and Madras to Chennai, if we didn't get an answer when we said Bombay, we would try Bombay, and it worked both ways If you first tried Mumbai, a non-Hindu wouldn't reply.

@Terry, yikes! Just one more thing for a busy pilot to worry about.

This is not correct. ATC procedures in the USA differ greatly from ICAO SARPS.


text:
What is the difference between point-to-point and low-cost airlines?
In describing major airlines, are "point-to-point" and "low-cost" synonymous? Or is this an over-generalization?

What is the difference between point-to-point and low-cost airlines?

Point to point describes an airline that focuses on delivering services without a connecting hub. This cannot be said to imply low cost.
A independent airline flying between some islands and not offering connections would fall into this category as much as Ryanair, Easyjet or Spirit Airlines.
Low Cost generally implies a point-to-point operation. They structure their business model around filling aircraft on high-demand routes.
Reasons for this: The cost and complexity of handling and organising hubs and connecting flights violates their carefully controlled profit margins and structure.
Hence IMHO you can normally assume a low-cost operation will be a point-to-point operation, but not the other way around.

Several low cost carrier does have connecting hub like Eurowing and Airasia


text:
Are flight attendants paid in the same way pilots are paid?
I understand that airline pilots are generally paid for their time "in flight", which means the clock starts when the aircraft begins to move away from the gate. Are flight attendants paid in the same way?

I am VtC as too broad, unless the OP narrows this down to one (or at most a few) jurisdictions.

Considering that the actual work of a pilot starts way before moving the plane, I'm kind of surprised to hear that. Maybe when cutting costs they'll refuse to pay any time on autopilot ;-)

It mightn't be a bad idea to specify which country you are talking about, as practices can vary throughout the world, and potentially even amongst individual airlines within the same country.
Last year I looked into becoming a FA for a smaller Australian airline. The pay arrangements were:

A guaranteed minimum annual salary (which was about $45k)
On top of that,  flying wages based on scheduled flight times (4.20 per hour, a bit extra per hour for the hours worked above an agreed number of hours in a month). Note that even if a flight is delayed it is still issued with adjusted scheduled times. The 'padding' applied to regular scheduling works in favour for the employees here.

However, I believe the US  works completely differently. And as I said, this is an example from only one airline.

It really all depends on the airline and their CBAs with various unions. Practically every major job role you encounter while taking a plane trip is represented in the U.S. by a different trade union; there's a pilot's union (several actually, biggest is the ALPA but some major carriers like AA and USAir have their own), a flight attendants' union (AFPA and AFA-CWA plus some carrier-specific affiliates), a mechanics' union (AMA and AMFA), a ground crew's union (mainly AFL-CIO or TWU), a baggage handlers' union (also either TWU or AFL-CIO but it can differ from the one for maintenance crews), an ATC union (NATCA), and even the TSA screeners are now unionized through the American Federation of Government Employees.
All of these unions negotiate more or less independently with the airlines on many things, including pay structure. It's often a matter of who gets there first when there are rumors of increased airline profits; on one airline the flight attendants might be treated like gold while the pilots are making peanuts and pushed right up against FAA regs on working hours and conditions, while the exact opposite may be true just one gate down. Exactly how they're compensated is usually public information since the airlines themselves are publicly traded and this information is part of their cost structure, but it is more likely to be different between airlines than the same, especially for a major U.S. carrier whose flight crews are represented by company-specific unions.


text:
Do pilots have a generally preferred "traffic" resolution?
For aircraft cruising along at altitude, it is common for the controller to assign an aircraft a new course or altitude or speed for some reason such as "for traffic". My question is, would most turbojet/turboprop pilots rather have a vector, say 20 degrees off course for 10 minutes, or have a different altitude, or have a speed assignment? What is most efficient for the airplane? Do companies have a preference? Are certain actions easier for the pilot? I suppose there is a fuel-use vs. time-en-route trade off in some cases, and many other factors as well, and personal preferences might vary widely, or maybe people don't really care?

Related

If ATC offered me a heading change or an altitude change and I was happy with the altitude I was at, I'd take the heading change in a heartbeat.  Compare:
Altitude change:

Dial the alt preselect up.
Select speed or VS mode (usually speed).
Set climb thrust.
Monitor the capture.
As the AP captures and pitches over, start bringing the thrust back to keep from speeding up until the original thrust setting is restored.

Heading change:

Select Heading Mode if in Nav mode.  If on vectors, it's already set.
Twirl the heading bug and monitor the turn.
When told to, dial the heading bug to a new heading as instructed or to a suitable intercept angle and reset Nav mode if told to resume self navigation.

Notice that with the heading change I don't have to screw around with power, which can be more hassle or less hassle depending on if you have FADEC, but in any case a lot more work than twirling the heading knob and pushing a button.

Not to mention the fuel savings of staying at altitude. That's the biggest factor in my mind.

There are a few kinds of traffic advisories but generally pilots don't (and can't) have a preference at least not one they can generally act on. Keeping routes as short as possible and staying on time is what any pilot wants but you cant always get what you want!
Those that come from TCAS require a quick climb or dive and preference is irrelevant you do what the system says.
As for ATC routing changes are expected, you are required to follow them, and generally there is no preference. For traffic alerts under VFR flight following you may get a

Turn right 20 degrees for climbing traffic

This is a fairly benign maneuver and generally you will get some kind of return on course not to long after. Turning left or right is all the same and on anything bigger than a GA plane its likely handled by the turn of a dial on the auto-pilot.
In crowded terminal approach areas changes, and quick ones, are to be expected. Pilots cant really have a preference as they don't really have a full picture of whats going on. ATC is there to route planes, not suggest what they should do. If you want a bit of an over dramatic representation I would advise watching Pushing Tin if you want some real examples there are lots of videos out there of the real thing and you can stream it here if you want to hear it in real time.
Weather diversions are a bit more complicated especially in aircraft that are equipped with weather radar. When it comes to weather diversions ATC has a lot of info but the aircraft may have actual situational info. In this case the aircraft can request a diversion if they see something crop up and ATC may issue advisories and make a diversion available. In this case the pilot may have some preference based on what they are seeing on their onboard radar.

Fuel usage is something to consider any time a diversion is issued. Generally pilots can not denny a diversion but no pilot should willingly create a dangerous situation and diverting without taking fuel into account is potentially dangerous.

Why should pilots have no preference? What you really mean is that pilot's preference for traffic resolution is not relevant.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How turbercle wings could generate aerodynamic advantage?
Recently it has been discovered that turbercles present in fins from whales are actually providing hydrodynamic benefit for them.
As well, it has been proposed that this effect will provide benefit for airplanes.

From my perspective I can see strong difficulties to see this design able to improve current leading edges, as well, is a design working in water, in a lower Reynolds environment.
My question is actually three:

What is the aerodyamic (or hydrodynamic) mechanism and benefit that these turbercle provide?
How applicable is that to the airplane industry as proposed in several publications? (first thing I see difficulties is the existance of high lift slats and compatibility with these turbercles, probably more applicable to tails).
Which are the potential drawbacks of this design?

probably an extension of shock bodies, Owls have similar ridges on the leading edge of the flight feathers

relevant wiki page is a stub with a reference to a pdf paper with some discussion about a set of computer simulations.

@ratchetfreak I read that paper prior to the question and it was not conclusive, only addressing one angle of attack and poor investigation of viscid effects. Looks to me very academic.

@ratchetfreak: Last I checked, owls didn't usually fly at transonic speeds.

No, please, not another revolution of how we build airplanes again!
The linked article mentions two guys who pose as the discoverers of humps on whale flipper leading edges. What precisely have generations of marine biologists overlooked before?
Now to the humps: They produce a 3D flow pattern close to the leading edge which will help the flow to transition to a turbulent boundary layer more quickly. This is helpful at high angle of attack at intermediate Reynolds numbers (around 1 Mio) and will make the whales more maneuverable.
This is not to say that it will help any existing aircraft. Had nature given the whales the opportunity to integrate slats and double-slotted flaps into their flippers, they sure would have done so. This will help much more to increase maximum lift. Also, typical aircraft Reynolds numbers are much higher than those on whale flippers, and there turbulent transition is very quick. Right now, most efforts center on delaying this transition to reduce friction drag.
Swept wings will see a massive drag increase if this sort of leading edge is used.
The "tubercles" are a very specific form of turbulators and optimized for a narrow application. In other words, don't invest in this WhalePower company!

Is basically what I guessed. Do you have references of any study or further information for typical aircraft wings?

@TrebiaProject: That is a very general question. My experience is with gliders and different types of turbulators, less with studies. Look at zig-zag-tape for a comparable way to create a 3D disturbance which helps to trip transition when the pressure gradient is right.


text:
What is the difference between the NTSB's "Aviation Accident Database" and their "Aviation Accident Reports" list?
I'm trying to understand why there are thousands more records in the "Aviation Accident Database," located here:
https://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx
than there are in the "Aviation Accident Reports" list, located here:
https://www.ntsb.gov/investigations/AccidentReports/Pages/aviation.aspx
For example, in 1991 there are 2,462 records in the accidents database and only 8 records in the accident reports list. What determines whether or not an accident ends up in the list of reports? What is the difference between these two data sets?

The NTSB will investigate every aircraft accident in the US, and any that foreign authorities delegate to them. As you found, there thousands of accidents ever year. Most of these are not fatal. The NTSB will at least gather the factual information and do their best to arrive at a probable cause.
With limited resources, the NTSB must decide which accidents are important enough to do a major investigation. You can see a map and list of the most important current investigations here, along with the following explanation (emphasis mine):

This listing of investigations includes those open investigations involving accidents or incidents which resulted in significant loss of life, physical damage, involve issues of importance to public safety, or which have generated particular public interest. This represents only a small portion of the investigations conducted by the NTSB, most of which occur at the regional level.

There is no official criteria and not all of the incidents listed will become major investigations that result in full reports. The NTSB will decide based on the situation and factual information whether a full report will be produced. These reports are much more in depth, involve official parties that may submit their own findings and recommendations, and are deliberated on by the board in a public meeting before being officially released along with formal safety recommendations.
The board also periodically releases safety recommendation reports on issues that may not be related to a single major accident, but that are evidenced by multiple accidents that may not have received major reports.

Every civil aircraft accident in the U.S. (although they are sometimes asked to assist with investigations of military aircraft accidents).  Important distinction there.


text:
Is there any equation to bind velocity, thrust and power?
I am designing a remote controlled airship. I will tune it so that the lift given by Archimedes' Principle will exactly balance the weight of all the structure. It will be propelled by brushless motors with propellers on them.
As far as I have understood, for some given velocity $v$ the drag force $D$ will be given by the air pressure, some shape dependent drag coefficient $C_D$, the surface $S$ that the airship offers to the wind, and finally the squared velocity $v^2$.
Now, to keep some speed, obviously the thrust $T$ must equal the drag $D$. Now, I need to get to some equation for the power that I need to offer such thrust at such speed, given that I am using non-ideal engines, with non-ideal propellers etc. From a barely theoretical point of view, I know that if I want to apply some force to an object that moves at some speed, I will be using some power $P \sim Fv$. Now, since the thrust is generated in some way that actually looks very dispersive, I would like to know if there exists some relationship between the power $P$, the velocity $v$ and the thrust $T$, given some specific motor and some specific propeller. In particular, what are the parameters that one needs to know to get to this relation?
On example, I can imagine that some efficiency parameter for the engine, its RPM rate, its voltage, the diameter of the propeller, the pitch of the propeller, these are all going to be relevant to the equation I am looking for, but I wouldn't know how to explicitly figure out that.
It such a relation doesn't exist in an obvious or general way, could you just give me an idea of the efficiency of the propulsion? I mean, I know that $P \geq Tv$, but how much bigger is it in general? Do these quantities share the same order of magnitude, or the dispersion is very big compared to the actual propulsion?
As you can understand, I am not an expert at all on the subject, so I would appreciate anything that could get me started.
Thank you again!

A propeller accelerates the air of density $\rho$ which is flowing through the propeller disc of diameter $d_P$. This can be idealized as a stream tube going through the propeller disc:

The air speed ahead is $v_0 = v_{\infty}$ and the air speed aft of the propeller is $v_1 = v_0 + \Delta v$. The propeller effects a pressure change which sucks in the air ahead of it and pushes it out. Since the mass flow must be equal ahead and behind the propeller, the stream tube diameter is bigger ahead of the propeller and smaller downstream. In reality, there is no neat boundary between the air flowing through the propeller and that surrounding it, but for computing thrust this simplification works well if the airspeed is identical across the cross section of the propeller disc.
The mass flow (mass $m$ per unit of time $t$, written as a derivation) is: $$\frac{dm}{dt} = \pi \cdot\frac{d_P^2}{4}\cdot \rho \cdot \left( v_{\infty} + \frac{\Delta v}{2} \right)$$
The mass flow is written as the volume of air with density $\rho$ per time, moving through the propeller disk with the diameter $d_P$ at a speed that is the median between entry and exit speed. The thrust is mass flow times speed change: $$T = \pi \cdot\frac{d_P^2}{4}\cdot \rho \cdot \left( v_{\infty} + \frac{\Delta v}{2} \right) \cdot \Delta v$$
If the engine has the power P, the thrust is net power divided by airspeed in the propeller disk. To arrive at the net power, you multiply the rated engine power by the propeller efficiency $\eta_{Prop}$ and the electric efficiency $\eta_{el}$: $$T = \frac{P\cdot\eta_{Prop}\cdot\eta_{el}}{\left( v_{\infty} + \frac{\Delta v}{2} \right)}$$
A good engine will have an electric efficiency above 90%, and a good propeller will give you an efficiency between 80% and 85%. Efficiency goes up with lower $\Delta v$, so a big, slowly spinning prop is better than a small, fast one.
An airship will not move fast, so the $v_{\infty}$ is low. In the static thrust case it is zero, and the thrust equation can be simplified:
$$T_0 = \frac{P\cdot\eta_{Prop}\cdot\eta_{el}}{\sqrt{\frac{2\cdot T_0}{\pi\cdot d_P^2\cdot\rho}}} = \sqrt[\LARGE{3\:}]{P^2\cdot\eta_{Prop}^2\cdot\eta_{el}^2\cdot\pi\cdot \frac{d_P^2}{2}\cdot\rho}$$
To actually know your $v_{\infty}$, you need to know the drag $D$ of your airship. The general equation is $$D = A\cdot c_D\cdot\frac{\rho}{2}\cdot v^2$$ with $A$ the frontal area of the ship and $c_D$ its drag coefficient. S. Hörner (page 14-1) gives the drag coefficient of LZ126 (later Los Angeles) as $c_D = 0.023$ for the hull alone and $c_D = 0.071$ for the complete ship, including nacelles, fins and all.
A similar result is given in NACA Report 394 which documents the tests done on models of Goodyear Zeppelins in the NACA variable density wind tunnel in 1932. There is another source, NACA Report 117 by Max Munk, written in 1921 and supposedly collecting the results of German measurements on Zeppelins which can be found here.
Your model will not achieve such low values, however, because it will fly at a lower Reynolds number, which means that friction will be higher in relation to other forces. Depending on the size and speed of your model, pick a value between 0.15 and 0.3 for first calculations.

Never received such a clean and understandable answer in my whole stackexchange life. Thank you!

Amazing answer. I tried to calculate all this from first principles, with your guidance, in order to completely understand it. I only have one question. In the final step where you calculate thrust $T_o$, by substituting in $Δu$, i'm missing a number in my calculations. Since $ Δu=\sqrt{\frac{8T_o}{\pi d_p^2p}} $ so when you substitute in $\frac{Δu}{2}$ i get $ Δu=\sqrt{\frac{2T_o}{\pi d_p^2p}} $, instead of $ Δu=\sqrt{\frac{T_o}{\pi d_p^2p}} $ that you say..

@RestlessC0bra: Thank you very much for checking this! You're right, I forgot the 2. Fixed it.

Ah no problem. I was surprised by that mistake.

Hello Peter. Can you please elaborate a bit more on your statement that "a big, slowly spinning prop is better than a small, fast one"? I try to figure it out using the equations here, but for example if i reallocate the terms in this equation: $T = \frac{P\cdot n_{prop}\cdot n_{el}}{\Big(V_{\infty} +\frac{\Delta \upsilon}{2} \Big)}$ i get the opposite statement. What am i missing?

@RestlessC0bra: Better in this context means higher efficiency. A big propeller will capture more air and will need a smaller $\Delta v$ to produce the same thrust, and will consequently spin much slower. In mathematical terms: The denominator grows with $\Delta v$, so the thrust from a given power becomes higher. Now I only hope I have understood your question correctly.

@MatteoMonti: Now I could locate NACA TR 117 here


text:
How can two planes be on the same taxiway facing opposite to each other?
Curious to know how two planes can be on the same taxiway facing opposite to each other at the same time? Please have a look at the satellite view of the following coordinates in Google Maps:

(-27.369843, 153.126482)

Are you asking how they were physically placed there?

Anecdotal comment, this happens a lot in basic simulator training in ATC school. They are typically facing towards eachother though, not away from eachother. And you'll usually hear some loud swearing when it happens ;)

In answer to the "How can they be portion" - they were pushed and/or pulled there by tugs. The real question is why the heck are they parked in the middle of the taxiway instead of near one end or the other.

Possible duplicate of How does Boeing park several airplanes on a single runway at Paine Field?

That's how little Cessnas are made.

Well, not a frequent occurrence (happily), but it can happen if you're taxiing at Chicago O'Hare on your first time there (and thus unfamiliar with the taxiways) and you pass a taxiway that you should have turned on. Fortunately the aircraft coming towards us had a taxiway coming up to turn on, thus preventing the need to call out a tug (or power back?). That, however, did not stop the ground controller from telling me in no uncertain terms that I had just screwed up big time as he re-cleared both of us to get where we should be going.

The taxiway is being used for parking. Specifically, parking of two MD-80s that have attracted press attention—The Australian: Clive Palmer’s $10m in tax-haven jets sitting idle:

TWO 155-seat jets owned by Clive Palmer and branded with the name of
  his controversial and longtime loss-making company Mineralogy sit
  mothballed in a remote corner of Brisbane airport.
The McDonnell Douglas MD-82 jets, worth roughly $5 million each, are
  registered in the Cayman Islands tax haven and have sat mothballed for
  months, possibly years.
...
A spokeswoman from Brisbane airport confirmed the planes were owned by Mr Palmer and had sat idle for “at least several months”, but was unable to confirm exactly how long. “The space is leased from Brisbane airport and the owner pays leasing fees to have them there,” she said.

That article from 2014 shows them parked head-to-tail, but presumably they've been moved since then (the Google maps imagery is from 2017).
If you're wondering how I found this, I zoomed in on the satellite image and used 3D mode to get a better view of the side of the aircraft well enough to sort of make out the "Mineralogy" label on the side, and Googled from there.

Good find.  It looks like YBBN moves them around.  In the current mapping imagery, they are shown in two places!

Great detective skills

More like terrifying detective skills.  I'm really scared of satellites now =p

@CortAmmon Google maps "satellite" view is actually from aerial photography.

@CortAmmon you should really be scared of Google & their "street view" images, and I'm pretty sure they put the "street view" cameras on some relatively low flying airplanes to spruce up their 3D models

Without knowing the exact date and time that the images were taken and with access to someone on shift at the time, it's impossible to give a definitive answer so here are some "myth plausibles".

Mapping images are stitched together from different frames from different satellite passes by software.  The software will select two images based on contrast, clarity and other optical attributes or perhaps because of data corruption.  Whilst this image is made up of at least two frames from different times, I can see no signs that the two aircraft are from different frames.
A controller made a mistake and cleared two aircraft onto the same taxiway at the same time.  It's happened before, it will happen again. However, these are tail to tail and I cant think of anyway that this could have happened.
The only remaining explanation I can think of, and the evidence points to this being the case, is that the taxiway is being used for temporary parking. The ramp is pretty full and it is plausible that the two aircraft have been parked tail to tail so that either one can get out without having to move the other.

Searching Google shows regular NOTAMs which support this, e.g.

J0899/13 NOTAMN         [ Taxiway ]
    Q)YBBB/QMXAU/IV/NBO/A/000/999/2723S15307E
    A)YBBN B)1303140318 C)1306130700EST
    E)TWY F3 NOT AVBL DUE ACFT PARKING**

Is your point 2 specific to the situation in the question? There's no problem having multiple airplanes on a taxiway as long as they're headed in the same direction.

@fooot No.  I mean that two aircraft have ended up nose to  nose (or at least, going in opposite directions) on the same twy. So yes, it's possible (and normal if going the same way) for two aircraft to taxi on the same twy but not tail to  tail!

The image is real they have been parked that way for several years and have not moved I know this because they are at the General Aviation Terminal at Brisbane airport and I go by them fairly regularly there is no air controller errors of trick photography anyone looking out the window flying in and out of Brisbane Domestic will see them sitting there.


text:
How much data required in the 747 design?
I am interest in how much computer data is required in building the 747>

What? I assume you're asking about the amount of research used to design the 747 in the first place. We definitely can't provide a clear answer to that. It's reasonable to say that, directly or indirectly, the majority of all aviation literature would have been read by at least one of the designers on the team.

What sort of answer were you looking for? "57.3 TB"?

The 747 was designed in the 1960s (rollout in September 1968) so a rather simplistic answer to "how much computer data..." is none. The design would have been done on manual drafting tables using slide rules and mechanical calculators, with actual computers only used for the occasional complex calculation assuming they didn't crash in the process. These calculations would have produced a single value or small set of values, not a finished design by modern standards
You are possibly thinking of the 777 design process. This was the first Boeing product designed entirely with CAD systems. They use CATIA by Dassault, the details are not readily available but I think we can assume that the amount of data involved is Large.


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What fraction of air intake becomes bleed air in airliner engines?
Gas turbines typically bleed air from the late compressor stages. Some of this air bypasses the combustion chamber and is expanded through the turbine to prevent the blades from becoming too hot. I believe bleed air is also used to provide hydraulic control of the control systems and also for utilities such as air conditioning and electricity generation.
I am interested to know just how much of the air intake to the gas turbine compressor becomes bleed air. I'm also interested in knowing how much of this bleed air goes to each application e.g. Utilities vs. turbine cooling vs. hydraulic control.
I suspect it varies a fair bit so I am mostly interested in how this occurs on larger airliners that would typically fly long international flights.

See https://aviation.stackexchange.com/questions/31782/how-much-is-air-mass-flow-rate-of-bleed-air-used-for-environmental-control-syste/42905#42905 Answers part of but not all you questions.

Indeed about 1-2% of compressed air is bled off for cooling turbine disks and blade roots. When turbine inlet temperatures are higher than 1350K, the turbine blades must be internally cooled as well, and up to 15% of compressor air may be required as bleed air for cooling. Source

Bleed air is also used for aircraft systems. If we take a typical airliner such as the B737NG, compressed air can be supplied by either the engines, the APU or an external air chart, and is used for the following purposes:

Air conditioning & cabin pressurisation, which are both subject to regulation quality standards.
Wing & engine thermal anti-icing
Engine starting
Hydraulic reservoir pressurisation. The reservoirs are pressurised to ensure that hydraulic fuel reaches the hydraulic pumps.
Water tank pressurisation

Compressed air is tapped from the engine 5th and/or 9th compressor stage. Air supply is regulated according to demand and varies according to flight stage - during flight stages with relatively high engine power (take-off, climb, most cruise), 5th stage compressed air only is used. During low engine power conditions such as approach and landing, 9th stage compressed air is used for the pneumatic systems as well. This air is under higher pressure, and hotter, than the 5th stage air.

Exactly how much bleed air is used for the systems is hard to get a reference for, but here is an estimate for the CFM56-7B18:

Bypass ratio = 5.5:1, Air mass flow = 307 kg/s => hot flow = 307 / 5.5 = 55.8 kg/s
Cabin air systems refresh the whole cabin volume every 2-3 mins. Let's take 2 minutes, without recirculation.
Internal volume of a 737NG is about 400 $m^3$ (from fuselage width and length of an -800).
Air density = 0.94 kg/$m^3$ (we'll take cruise conditions, cabin air density @ 2,000 m)
Aircon flow = 400 * 0.94 / 120 = 3.1 kg/s

There are two engines, so the percentage of flow used for the aircon and pressurisation would be 3.1 / (2 * 55.8) = 2.8% maximum. Please again note that this is a ROM value which I will gladly give up for an official reference.
As a magnitude check: the APU brochure mentions 154 pounds of air per minute = 1.16 kg/s for environmental control and engine start. The APU can deliver air conditioning on the ground without the engines running so that would be the right order of magnitude.
Final estimate: bleed air for air conditioning is between 1 - 3 kg/s is about 1 - 3% per engine.
Bleed air is not used for:

Hydraulic power generation. The hydraulic pumps are engine driven pumps (via a gearbox) and electrical pumps, powered by the AC system
Electrical power generation. This is provided by Integrated Drive Generators, from a generator in the APU, and from the batteries through an inverter.

Source: own work from open source on-line pilot training courses.

You didn't say how much bleed air for a/c, etc

@TomMcW You're right, I made an estimate.

Cool. +1 I've always wondered this myself. I knew when they needed some extra thrust the pilots could cut bleed air, but I never knew how much they could squeeze out

I like the approach for a ROM, however, isn't there a large degree of recirculation of the cabin air? https://aerospace.pall.com/en/commercial-fixed-wing/how-cabin-air-systems-work.html says 50%. Does that needs to be allowed for in the calc?

@Richard yes there is, there is quite some variation with number of passengers, set temperature, recirculation etc. My ROM was for a maximum demand scenario, it seems to be on the high side judging from the APU capacity.

Internal cooling air might be better termed bypass air as it is not actually bled out of the engine so the energy of compressing this bypass air remains within the overall cycle rather than draining substantial energy from the system, and because turbine combustion is normally oxygen rich this bypassed air does not reduce the total fuel that can be burned.
The pilots can shutoff all accessory bleed air for improved engine performance on takeoff and low altitude climbs. So 0% is the lower limit for bleed air.
Accessory bleed air is used mainly for cabin pressure, engine starting, and deicing. Pressurizing reservoirs is an extremely minor use and only needed at higher altitudes to reduce the chance of pump cavitation and foaming from dissolved gasses.


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Why can't jet engines operate with supersonic air and how do they slow it down?
Typically jets cannot operate when intake airflow is supersonic relative to the engine. Why is this so? Also, why are scramjets able to use supersonic air?
To slow down the air to subsonic speeds, the air passes through a shockwave (if I understand correctly). How does this slow down the air?

To avoid shock waves on the compressor blades which would make the engine unusable both because of the very large pressure fluctuations that would cause fatigue and failure of blades and because of the high level of drag that is developed in supersonic flows that would have the effect of slowing the blades down as they rotated. In fact, the engine just wouldn't run with a supersonic flow going into it.
Also, the flow needs to be slowed down as much as possible to allow enough time in the combustion chamber for the fuel to burn completely.
So... a cone or ramp shape at the inlet is used to create a little shockwave in front of the engine slowing down the incoming air to subsonic speeds and allowing the jet engine to operate efficiently.
A ramjet is able to use the compressed air because it is designed to do so. An excellent case study is the SR-71 Blackbird, which had engine cones that moved forward and backward based on speed/altitude, to transition from a turbine to a ramjet mission profile. (Fun fact: That plane is so darned fast that the limit to its speed comes not from engine power, but from MELTING THE PLANE because it is going so fast.) The SR-71 had "Bypass doors" to close of the main turbine of the engine when operating on a ramjet profile.
A ramjet sometimes referred to as a flying stovepipe or an athodyd, is a form of airbreathing jet engine that uses the engine's forward motion to compress incoming air without a rotary compressor. Ramjets cannot produce thrust at zero airspeeds; they cannot move an aircraft from a standstill. A ramjet powered vehicle, therefore, requires an assisted take off like a JATO to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3. This type of engine can operate up to speeds of Mach 6.

A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to forcefully compress the incoming air before combustion, but a ramjet decelerates the air to subsonic velocities before combustion, while airflow in a scramjet is supersonic throughout the entire engine. This allows the scramjet to operate efficiently at extremely high speeds: theoretical projections place the top speed of a scramjet between Mach 12 and Mach 24.

How does the shock wave slow the air down? Does it simply apply a force to the air (much like an explosive blast would) which slows it down? Or is there some more complicated underlying principle?

@HCBPshenanigans: Well, it is somewhat complicated. Bernoulli principle says restricting flow reduces pressure and increases speed, but that only holds for incompressible fluids and for supersonic restricting the flow increases pressure and decreases speed.

read any of the SR-71 manuals for a great discussion on operating supersonic engines

Not Bernoulli, but Hugoniot is the right name here. See the Wikipedia page for the Hugoniot equation for details.

I read that adding heat to a flow with a Mach number greater than 1 will decrease the Mach number toward 1, and adding heat to a Mach number less than 1 will increase the velocity.
If this is true, wouldn't it set a limit to the speed of a jet engine?
Or, do engines increase the heat, slowing down flow, and then increase the flow speed again using nozzle shape?

In a nutshell
A compressor blade works best in subsonic flow. Supersonic flow introduces additional drag sources which should be avoided if efficiency is important. Thus, the intake has to slow down the air to a Mach number between 0.4 and 0.5. Note that the high circumferential speed of a large fan blade will still mean that its tips work at around Mach 1.5, but the subsequent compressor stages will operate in subsonic conditions.
A scramjet is possible with fuels with supersonic flame front speeds and rapid mixing of fuel and air. If the engine would burn regular kerosene, the flame would be blown out like a candle if the internal airspeed would be supersonic, and even if flame holders keep the flame in place, most combustion would take place only after the fuel-air mixture has left the engine due to the slow mixing of kerosene and air. By using hydrogen, a stable combustion can be achieved even in supersonic flow. Due to the high flight speeds, compression is possible by a cascade of shocks, so no moving turbomachinery is needed in ramjets and scramjets.
Background: Maximum heating of air
All jets decelerate air in their intake in order to increase air pressure. This compression heats the air, and in order to achieve a combustion which produces thrust, this heating must be restricted. If air is heated above approx. 6,000° K, adding more energy will result in dissociation of the gas with little further heat increase. Since thrust is produced by expanding air through heating, burning air that enters the combustion process already at 6,000° K will not achieve much thrust. If the air enters the intake at Mach 6, it must not be decelerated below approx. Mach 2 to still achieve combustion with a meaningful temperature increase - that is why scramjets are used in hypersonic vehicles.
Full disclosure: Oxygen starts to dissociate already between 2,000° and 4,000° K, depending on pressure, while Nitrogen will dissociate mainly above 8,000° K. The 6,000° K figure above is a rough compromise for the boundary where adding more energy starts to make less and less sense. Of course, even a 6,000° K flame temperature is a challenge for the materials of the combustion chamber, and ceramics with film cooling are mandatory.
The equation for the stagnation temperature $T_0$ of air shows how important the flight speed $v$ is: $$T_0 = T_{\infty} \cdot \frac{v^2}{c_p} = T_{\infty} \cdot \left(1 + \frac{\kappa - 1}{2}\cdot Ma^2 \right)$$
$T_{\infty}$ is the ambient temperature, $c_p$ the specific heat at constant pressure and $\kappa$ the ratio of specific heats. For two-atomic gases (like oxygen and nitrogen), $\kappa$ is 1.405. Temperature increases with the square of flight speed, so at Mach 2 the factor of heat increase over ambient is only 3.8, while at Mach 6 this becomes 26.3. Even at 220° K air temperature, the air will be heated to 5,800° K when it is ideally compressed in case of a hypersonic vehicle traveling at Mach 6. Note that real compression processes will heat air even more due to friction.
Compression with shocks
Supersonic flow is slowed down by a pressure rise along the flow path. Since no "advance warning" of what is coming is possible, this pressure rise is sudden: Pressure jumps from a fixed value ahead to a higher, fixed value past the jump. This is called a shock. The energy for the pressure rise is taken from the kinetic energy of the air, so past the shock all other parameters (speed, density and temperature) take on new values.

F-16 air intake (picture source)
The simplest shock is a straight shock. This can be found at the face of pitot intakes like the one of the F-16 (see the picture above) in supersonic flight. More common are oblique shocks which are tilted according to the Mach number of the free flow. They happen on leading and trailing edges, fuselage noses and contour changes in general: Whenever something bends the airflow due to its displacement effect, the mechanism for this bending of the flowpath is an oblique shock.

straight and oblique shock (own work)
The index 1 denotes conditions ahead of the shock, and 2 those downstream of the shock. For weak straight shocks the product of the speed ahead of the shock $v_1$ and the speed past the shock $v_2$ equals the square of the speed of sound: $$v_1\cdot v_2 = a^2$$
If $Ma_1 > 1$, then $Ma_2$ must be smaller than 1, so the flow is always decelerated to subsonic speed by a straight shock.
The same equation works for the normal speed component $v_n$ ahead and past a weak oblique shock: $$v_{1n}\cdot v_{2n} = a^2$$
Note that the tangential component $v_t$ is unaffected by the shock! Only the normal component is reduced. Now the speed $v_2$ is still supersonic, but lower than $v_1$, so a weak oblique shock produces a modest increase of pressure, density and temperature.
The angle of the oblique shock wave is determined by the Mach number ahead of the shock.
Supersonic intakes
Weak shocks are desired, because they produce only small losses due to friction. Pitot intakes with their single, straight shocks work well at low supersonic speeds, but incur higher losses at higher Mach numbers. As a rule of thumb, a pitot intake is the best compromise at speeds below Mach 1.6. If the design airspeed is higher, more complex and heavier intakes are needed to decelerate the air efficiently. This is done by a sequence of weak, oblique shocks and by means of a wedge intake. The picture below shows the intake of the supersonic Concorde airliner:

Concorde intake (picture source)
Gradually increasing the angle of the wedge is causing a cascade of ever steeper, oblique shocks which gradually decelerate the air. The design goal is to position this cascade of shocks caused by the wedge on top such that they hit the lower intake lip. This is done by a moveable contour of the upper intake geometry and/or the lip. The goal is to achieve a uniform speed over the intake cross section and not to waste any of the compressed air to the flow around the intake. See the picture of the Eurofighter intake below for an example of a moveable intake lip (which admittedly is mainly for increasing the capture area at low speed and for avoiding flow separation even with a small intake lip radius).

Eurofighter intake (picture source)
Once the air has entered the intake, it is only mildly supersonic and can be further decelerated by a final, straight shock at the narrowest point of the intake. After that point, the intake contour is gradually widened, such that the air decelerates further without separation. To achieve this, a very even flow across the intake area is mandatory, and even the slight disturbance caused by the boundary layer of anything which is ahead of the intake must be avoided. This is achieved by a splitter plate which is clearly visible in the pictures of the F-16 and Eurofighter intakes. The splitter plate of the Eurofighter intake is even perforated to suck away the early boundary layer there.
The deceleration of the intake flow results in a significant pressure rise: In case of the Concorde at Mach 2.02 cruise, the intake caused a pressure rise by a factor of more than 6, so the engine compressor had to add "only" a factor of 12, such that the pressure in the combustion chamber of the four Olympus 593 engines was 80 times that of the ambient pressure (admittedly, this ambient pressure was only 76 mbar in the cruise altitude of 18 km).
This pressure increase means that a supersonic intake must be built like a pressure vessel, and the rectangular face of the intake must quickly be changed to a round cross section downstream to keep the mass of the intake structure low.
Intakes at higher speed
Going faster means the intake pressure recovery increases with the square of flight speed: In case of the SR-71 intake at Mach 3.2, the pressure at the engine face was already almost 40 times higher than the ambient pressure. Now it becomes clear that going faster than Mach 3.5 does away with the need for a turbocompressor: At these speeds a properly designed intake can achieve enough compression by itself for the combustion to produce enough thrust, and going above Mach 5 will need restraint in slowing down the intake flow in order to have enough temperature margin for combustion, requiring supersonic flow in the combustion chamber.

Superb, concise and very informative answer!

Even if adding heat to air hotter than 6000K would cause it to dissociate rather than heating it up further, wouldn't that still increase the thrust of the engine (by increasing the combustion chamber pressure and thus the speed at which the superheated gas goes out the tailpipe)?

@Sean: All energy that goes into ionisation will not further expand the gas and will be wasted for propulsion. Thrust is generated by accelerating the gas, and this acceleration happens because the gas expands when heated.

@PeterKämpf: So why didn't you say "ionisation"?  I thought you meant "dissociation", which is when the molecules in the air break apart into individual atoms.

@Sean It is both. Molecules are stripped of their electrons and fall apart.

@PeterKämpf: Wouldn't the breaking-apart-into-atoms part still cause an increase in thrust, then?

@Sean: Yes, a bit indeed. But bought for a very uncompetitive price.

Is $v_{1n}\cdot v_{2n} = a^2$ the dot product of two vectors $v_{1n}, v_{2n}$ or just the arithmetic product of two speed magnitudes $v_{1n}, v_{2n}$ which would be two positive numbers? How is this formula derived?

@Hans: The product of two scalars. For more I recommend NACA Report 11358, look for Prandtl's relation

Why can't jet engines operate with supersonic air?

"Because there has been no business case to develop an engine with supersonic flow at entry to the compressor." The advantages would be the same that led to todays transonic (supersonic relative flow over part of the blade span) compressors, ie smaller and lighter. Compressors with supersonic relative flow over the whole blade span have been rig-tested at steady-state speeds, eg see Naca RM E55A27. Problems to be addressed (there are many) would include shock-induced boundary-layer thickening and separation in the compressor blade passages which causes unacceptably high loss of the potentially "useful" energy that the compressor rotor is putting into the air (there would be too much temperature rise and not enough density and pressure rise)
However, they can and do operate with supersonic air, but only over the outer portion of fan and core compressor front stages. Note this air is only supersonic relative to the fast-spinning rotor blades and is self-generated within the engine, ie is not received as supersonic air from the intake (see reason that air leaving intake and entering engine is subsonic in following answer).
The compressor's job is to compress and so the rotor, having first grabbed the air and whirled it around at high speed, also has to slow it down within the passage between the spinning rotor blades (and also through the following stator vane passages), ie it has to compress it if it wants to be called a compressor (no slowing down would mean no increase in pressure). The  compressor rotor blade profiles and the diverging area of the passages between them give rise to the type of shock waves that have subsonic flow behind them. The shock waves that are the natural mechanism to get from supersonic to subsonic flow interact with the blade boundary layers and bl thickening and separation means high losses and losses are what the compressor efficiency is a measure of. So everything has to be done with as much finesse as possible to minimise the effects of BL separation and that means limiting the Mach number of the air relative to the blades to low supersonic values and these occur where the blade speed is highest, ie at the tips.

How do jet engines slow supersonic air down?

The question asks how does the engine slow the air down. It is often said that the intake slows the air down. However, the air is going to slow down anyway, with or without an intake. The air flow through the engine, and hence subsonic velocity at entry to the compressor, is set in the first instance by the pilot's request, ie compressor speed/fuel flow. At supersonic speed, if there is no intake, the air slows down to the subsonic entry speed through a plane shockwave. To improve the 'overall pressure-ratio' part of the engine efficiency an intake is added which is a more efficient supersonic compressor than the free-stream, ie it has features which produce a higher ram rise at compressor entry and less spillage drag round the outside of the engine (see later when it doesn't) This requirement is extreme at high supersonic speeds and is the reason for ramps/cones/lip shaping before entry and more ramps/cones/boundary layer bleed/duct shaping inside the intake.
When the intake doesn't do its job. This occurred many times when flying YF12 and SR71 aircraft at high supersonic speeds. In a split second the intake would increase the total pressure loss of the air entering the compressor from its low design value of about 20% to about 70%. The intake had changed (ie unstarted) from being an efficient supersonic intake to being the most inefficient type possible, ie a pitot intake with slowing down of the air from Mach 3 to subsonic in one violent step instead of a number of gentler ones.
The air in the intake slows down "because the engine has controlling areas inside the engine which set the average axial speed of the air through the engine (which has to be low to keep pressure losses to an acceptably low level) and hence at entry to the engine and this speed is subsonic".
High air speeds only occur where energy exchange is taking place, ie from the compressor rotors to the incoming air and from the outgoing combustion gases to the turbine, and where the low Mach number flow in the jetpipe ( it's low to keep the pressure losses to an acceptable value) accelerates to sonic speed at the nozzle throat.
Controlling areas are the throat areas of the turbine nozzle guide vanes and the exhaust nozzle where the gas Mach number is 1 and cannot go higher.
As stated in a previous answer, the low air speed requirement through the combustor sets the air speed at entry to the compressor. From this subsonic flow the compressor can generate its own supersonic flow relative to its rotor blades if it is driven fast enough by its turbine.

Besides the fact that beyond those 6000K combustion does not provide much expansion is also the fact that decelerating the flow to subsonic increases the drag of the engine because shock waves are not reversible and therefore the pressure is not recovered at the back (imagine a shut off engine with inner subsonic flow travelling at that speed, it would have a high drag due to shock waves). At hypersonic speeds overcoming that drag on top of the airframe drag would be a no-no.
That is why I doubt whether the solution for the SABRE engine (you can google it), which has an internal subsonic flow, may be feasible even if it achieves a high degree of cooling before reaching the compressor.


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Is there a set of laws which govern international flights?
A few years ago, my brother and I were aboard an Emirates flight from Australia to New Zealand. At the time, I was 16 and he was 14.
At one point in the flight we were both offered alcoholic drinks. Considering the drinking age where I am from is 18, and ID is required if you look under 25, this came as quite a surprise to the both of us.
Considering different countries have different laws and aircraft often have multinational crew and passengers, what laws govern international flights?

The question Danny linked answers the title question. However, one other important thing to bear in mind is that you may also still be subject to some of the laws in your country of citizenship, even when you're abroad. I'm now aware of any jurisdictions in which this would apply specifically to drinking, but some countries may indeed prosecute you when you come home if they find out about certain things you've done overseas. Laws against sex tourism are an example of this.

@reirab You may want to put this comment, or an answer/edit in the original question if you think that it will help clarify something there.

International airlines follow the law of their country of registration. Hence your flight limits you based on the drinking age in Abu Dhabi, which is 18.
Note that some airlines, such as Turkish, have a policy to not serve alcohol on certain flights. On my experience with a Hong Kong airline, the flight into Hong Kong only served juices where outbound we had Tsingtao :)
Something more to read on travel.SE : Is it legal to drink your own alcohol onboard a plane over international waters?

In addition: Air law on Britannica and Wikipedia (with a list of conventions).